Female orgasm has perplexed scientists, fuelled an equality movement and propelled Meg Ryan to fame. Now researchers say they might have found its evolutionary roots.

The purpose of the euphoric sensation has long puzzled scientists as it is not necessary for conception, and is often not experienced by women during sex itself. But scientists in the US have come up with an answer. Human female orgasm, they say, might be a spin-off from our evolutionary past, when the hormonal surges that accompany it were crucial for reproduction.

“It is important to stress that it didn’t look like the human female orgasm looks like now,” said Mihaela Pavličev, co-author of the study from Cincinnati children’s hospital. “We think that [the hormonal surge] is the core that was maybe modified further in humans.”

Writing in the journal JEZ-Molecular and Developmental Evolution, Pavličev and co-author Günter Wagner from Yale University describe how they delved into the anatomy and behaviour of a host of placental mammals to uncover the evolutionary origin of female orgasm, based on the hormonal surges associated with it.

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In mammals such as cats and rabbits, these surges occur during sex and play a crucial role in signalling for eggs to be released from the female’s ovaries. By contrast in a variety of other mammals, including humans and other primates, females ovulate spontaneously.

By tracing these mechanisms of ovulation across the evolutionary tree of mammals, the authors found that so-called “male-induced ovulation” predates spontaneous ovulation, with spontaneous ovulation likely to have arisen in the common ancestor of primates and rodents around 75 million years ago.

That, they say, suggests that human female orgasm could have its roots in a mechanism for the release of eggs during sex – a mechanism that became redundant with the evolution of spontaneous ovulation, with female orgasm potentially going on to acquire other roles.

“There is a lot of discussion about whether it could have any functions like in bonding behaviour and things like that – so we cannot exclude that it actually has co-opted some other function after it lost its function in reproduction,” said Pavličev.

The authors say the theory is backed up by a comparison of the position of the clitoris, which gives rise to orgasms, in different mammals. Species that rely on hormonal surges during sex to ovulate tend to have a clitoris inside or near the female sex canal, making it more likely to be stimulated during sex. But, the authors note, those that ovulate spontaneously have the clitoris further away.

That, says Pavličev, also helps to explain why many women do not have orgasms during sex itself. “It seems very natural not to automatically accompany orgasm with copulation,” she said.

David Puts from Pennsylvania State University said the new theory is plausible although, he stresses, it only looks at the hormonal component of female orgasm. “Of course, it is difficult or impossible to investigate sexual pleasure in nonhuman animals,” he said.

But, he adds, the new research goes a long way to boosting out understanding of the ancestral form of human female orgasm. “From there, we can explore how these components have changed over evolutionary time in response to putative selection pressures,” he said.

Elisabeth Lloyd, professor of biology at Indiana University and author of The Case of the Female Orgasm, also described the research as important. “I think they are on to some pretty interesting ideas tracing back what occurs today as orgasm and seeing where it came from,” she said.

But, says Lloyd, she is yet to be convinced by Wagner and Pavličev’s theory, not least because it does not take into account neurological and muscular aspects of human female orgasm. What’s more, she says, very little is known about orgasms in the females of other species.

Instead, Lloyd favours the theory that female orgasms are simply a happy quirk of embryo development. “It is only at eight weeks that there is a surge of male hormones that turns the genitals into male genitals,” she said. While men need to orgasm in order to transfer sperm, women, said Lloyd, end up with the nerve and muscle tissues for orgasm as “a fantastic bonus”.

“It all seems to be rather purposeless – except for the enjoyment, obviously,” she said. “It doesn’t mean it is not important, it just means it doesn’t have an evolutionary purpose.”